This invention relates to processes for making surface impregnated solid catalysts and sorbents, and to flue gas desulfurization processes using a surface impregnated sorbent.
Sulfur dioxide is a constituent of various waste gases. Among these are flue gas formed by the combustion of fossil fuels, off gases from various chemical and petroleum refining processes, and smelter gases. A major source of sulfur dioxide pollution of the atmosphere is flue gas from electric power plants. Such flue gas typically contains about 0.2 to about 0.3% by volume of sulfur dioxide, trace amounts of sulfur trioxide, some oxygen (i.e., about 1 to 4% by volume) due to the use of excess air in combustion and about 10 to 15% of carbon dioxide. Because of the harmful effects of sulfur dioxide, it is highly desirable that the discharge of sulfur dioxide into the atmosphere be held to a low level.
It is known that sulfur dioxide can be removed from flue gas by contacting the flue gas with a suitable solid sorbent comprising an active material for the selective removal of SO.sub.2 on a porous carrier. The active material is generally metal or metal oxide, such as copper or copper oxide, or a potassium oxide-vanadium pentoxide mixture, and the porous carrier is generally an essentially inert solid refractory material such as alumina, silica, and the like. Flue gas desulfurization processes using sorbents of this type are described, for example, in British Pat. No. 1,089,716 and in U.S. Pat. No. 3,501,897.
The active material in the usual flue gas desulfurization sorbent is uniformly distributed throughout the carrier particles. Such sorbents are typically produced by impregnating the porous carrier material with a solution of a salt which is decomposable to the desired active material (usually a metal oxide), and then calcining. Desulfurization is accomplished by passing flue gas containing sulfur dioxide through a bed of the sorbent until the amount of SO.sub.2 in the effluent reaches a predetermined level, e.g., 10% of the amount of SO.sub.2 in the entering gas, then regenerating the sorbent with a reducing gas.
The active material of the sorbent ordinarily is not completely utilized. That is, when the sulfur dioxide content in the effluent gas reaches the predetermined level, there is still unconverted metal oxide in the sorbent. One reason for this is that sulfur dioxide penetrates to the core of the sorbent particles only to a limited degree. The present invention provides a flue gas desulfurization process which uses a sorbent in which the active material is predominantly in an outer zone of the carrier particles near the external surface thereof. This permits more efficient utilization of the active material.
Processes for producing surface impregnated porous solid contact materials are known. The term "solid contact material" in this connection is used to denote both catalysts and solid sorbents. The term "surface impregnated" refers to solid contact materials comprising a particulate porous solid carrier and an active material which is disposed predominantly near the external surfaces of the carrier particles rather than uniformly throughout the carrier particles. Materials of this type are described, for example, in U.S. Pat. No. 2,746,936 and in British Pat. No. 642,970.
U.S. Pat. No. 2,746,936, for example, discloses the preparation of impregnated catalysts by partially filling the pores of the carrier with an inert blocking liquid, and then impregnating the carrier material with an impregnating solution, drying, and decomposing the impregnating solution solute (usually a metal salt) into the desired catalytically active material. Partial filling of the pores by the blocking liquid may be achieved in various ways, e.g., by evacuating the carrier material and using a quantity of blocking liquid which is less than the pore volume of the carrier, or by immersing the carrier in the blocking liquid for a length of time which results in partial rather than total filling of the pores of the carrier. The final catalyst is a particulate material in which the active catalytic material is located predominantly in the outer portion of the carrier material. Various carriers and various active materials can be used. The catalysts are disclosed as useful in catalytic cracking and other hydrocarbon conversion processes. A disadvantage of this process is that procedures for partially blocking the carrier pores suggested in this reference do not lend themselves readily to large scale operations.
British Pat. No. 642,970 discloses that catalyst particles having a greter concentration of active material near the surface than near the center can be obtained by uniformly soaking the particles in an impregnating solution and then rapidly drying the particles at 150.degree.-200.degree. C.
It is also possible to produce surface impregnated solid contact materials by spray coating calcined porous carrier particles with an impregnating solution containing the desired active material while tumbling, and then drying and calcining the impregnated carrier particles. The depth of impregnation tends to be variable, particularly when the carrier particles are in shapes that have inside surfaces such as Raschig rings, and there is no clear-cut line of demarcation between the impregnated outer zone and the substantially unimpregnated center core.